The present invention relates to a seat belt retractor for accommodating a webbing in such a manner as to allow the webbing to be retracted and withdrawn, and more particularly, to a seat belt retractor with a torsion bar for absorbing load exerted on a webbing when an occupant wears the webbing and the webbing is stopped from being withdrawn in the event of emergency, such as a vehicle collision, at which large deceleration is exerted on the vehicle.
Conventionally, a seat belt device installed in a vehicle, such as an automobile, prevents an occupant from jumping out of a vehicle seat by restraining the occupant with a webbing or seat belt thereof, thereby protecting the occupant in the event of emergency as mentioned above.
Such seat belt device is provided with a seat belt retractor for accommodating the webbing. When the webbing is not used, the webbing is fully wound on a reel of the retractor. When an occupant wears the webbing, the webbing is withdrawn to hold the occupant on the seat. In the seat belt retractor, a locking mechanism is activated in the event of emergency as mentioned above to stop the rotation of the reel in a belt unwinding direction, thereby preventing the webbing from being withdrawn. Therefore, the webbing can securely restrain and thus protect the occupant.
However, the conventional seat belt retractor of the seat belt device has a problem. That is, when the webbing restrains and protects the occupant, a large deceleration is exerted on the vehicle, so that the occupant tends to move forward due to a large inertia force. Accordingly, a large load is exerted on the webbing, and the occupant receives a large impact load from the webbing. Though this impact load is not so severe for the occupant, it is better to absorb the impact load.
Accordingly, a seat belt retractor which has a torsion bar to absorb the load exerted on a webbing in the event of emergency when the webbing is worn has been developed.
FIG. 10 is a vertical sectional view showing an example of the conventional seat belt retractor having the aforementioned torsion bar. In the drawing, reference numeral 1 designates a seat belt retractor, reference numeral 2 designates a U-like frame, reference numeral 3 designates a webbing, reference numeral 4 designates a reel which is rotatably supported between side walls of the U-like frame 2 and on which the webbing 3 is wound, reference numeral 5 designates a deceleration sensing means which detects a large deceleration of the vehicle in case of emergency mentioned above, reference numeral 6 designates a locking mechanism which is activated by the deceleration sensing means 5 to prevent the reel 4 from rotating in the webbing unwinding or withdrawing direction, reference numeral 7 designates a torsion bar loosely fitted and inserted in the axial direction in the center of the reel 4 and rotatably connecting the reel 4 and the locking mechanism 6, reference numeral 8 designates spring means for always biasing the reel 4 in the webbing winding direction through a bush 10 by the spring force of a spiral spring 9, reference numeral 11 designates a pretensioner which is activated in the case of emergency mentioned above for producing webbing winding torque, and reference numeral 12 designates a bush for transmitting the webbing winding torque of the pretensioner 11 to the reel 4.
The locking mechanism 6 includes a pawl holder 14 which can rotate together with a first torque transmitting portion 17 described later of the torsion bar 7 and pivotally holds a pawl 13, and a lock gear 6a which rotates together with the torsion bar 7 in a normal state and stops according to the activation of the deceleration sensing means 5 to produce rotation relative to the torsion bar 7 in emergency and engages the pawl 13 with one of internal teeth 19 of the side wall of the frame 2, thereby stopping the rotation of the pawl holder 14, i.e. the rotation of the reel 4 in the webbing unwinding direction. The pawl holder 14 has an external thread portion 15. Screwed onto the external thread portion 15 is a nut-like stopper member 16 which rotates together with the reel 4.
The torsion bar 7 has the first torque transmitting portion 17 which is fitted to the pawl holder 14 for preventing the relative rotation, and a second torque transmitting portion 18 which is fitted to the reel 4 for preventing the relative rotation.
The reel 4 is always biased by the spring force of the spring means 8 in the webbing winding direction through the bush 10, the torsion bar 7, the second torque transmitting portion 18 of the torsion bar 7, and the bush 12. When the pretensioner 11 is activated, the webbing winding torque produced by the pretensioner 11 is transmitted to the reel 4 through the bush 12, whereby the reel 4 winds up the webbing 3 for a predetermined amount.
In the conventional seat belt retractor 1 having the aforementioned structure, the webbing 3 is fully wound by the biasing force of the spring means 8 when it is not used. As the webbing 3 is withdrawn at a normal speed for wearing the webbing, the reel 4 rotates in the webbing unwinding direction, so that the webbing 3 is smoothly withdrawn. After a tongue (not shown) slidably fitted to the webbing 3 is inserted into and engaged with a buckle fixed to a vehicle body, an excess amount of the webbing 3 pulled outwardly is wound onto the reel 4 by the biasing force of the spring means 8 until the webbing 3 is fitted to the occupant""s body without a feeling of oppression.
In the event of emergency as mentioned above, the webbing winding torque produced by the pretensioner 11 is transmitted to the reel 4, so that the reel 4 winds up the webbing 3 for a predetermined amount, thus strongly restraining the occupant. On the other hand, the deceleration sensing means 5 is activated by large deceleration produced in the emergency to activate the locking mechanism 6. That is, the activation of the deceleration sensing means 5 prevents the rotation of the lock gear 6a in the webbing unwinding direction and then the pawl 13 of the locking mechanism 6 pivots and engages one of the internal teeth 19 of the side wall of the frame 2 so as to stop the rotation of the pawl holder 14 in the webbing unwinding direction. As a result of this, the reel 4 rotates in the webbing unwinding direction relative to the pawl holder 14 while the torsion bar 7 is twisted. From this, the reel 4 rotates in the webbing unwinding direction while twisting the torsion bar 7. By this twisting of the torsion bar 7, the load applied to the webbing 3 is limited, thus absorbing the impact on the occupant.
Since the reel 4 rotates relative to the pawl holder 14, the stopper member 16 rotates relative to the external thread portion 15 which is screwed into the stopper member 16, whereby the stopper member 16 moves toward the pawl holder 14. As the stopper member 16 comes to contact the pawl holder 14, the stopper member 16 is prevented from further rotating, so that the reel 4 is also prevented from rotating and the torsion bar 7 is prevented from being twisted. In this manner, the withdrawal of the webbing 3 is stopped, whereby the occupant is restrained by the webbing 3 securely, and the maximum allowable twisting deformation of the torsion bar 7 is defined, thereby preventing the torsion bar 7 from being broken by further twisting.
In addition, in this conventional seat belt retractor 1, the pawl holder 14 of the locking mechanism 6 rotates in the webbing unwinding direction relative to the lock gear 6a when the webbing is rapidly withdrawn. Also in this case, the pawl 13 of the locking mechanism 6 engages one of the internal teeth 19 of the side wall of the frame 2 so as to stop the rotation of the pawl holder 14 in the same manner as mentioned above. Therefore, the reel 4 is prevented from rotating in the unwinding direction through the torsion bar 7, thus stopping the withdrawal of the webbing.
By the way, in this conventional seat belt retractor 1, the limited load is defined only by the torsion bar 7, and kinetic energy of the occupant can be absorbed just by the one limited load. However, the magnitude of the produced kinetic energy varies depending on the condition including the type of vehicle and the occupant""s body size. By the aforementioned structure in which the kinetic energy is absorbed just by the one limited load, however, it is difficult to suitably and effectively comply with the various conditions of the kinetic energy by controlling the limited load corresponding to the condition of the kinetic energy.
The present invention has been made under the aforementioned circumstances, and the object of the present invention is to provide a seat belt retractor, wherein limited load can be variously set to effectively absorb kinetic energy of an occupant.
To solve the aforementioned problems, a seat belt retractor of the first aspect comprises, at least, a reel for winding up a webbing; locking means having a locking member, rotation of the locking member in the webbing unwinding direction being stopped in the event of emergency; and webbing load limiting means rotationally connecting the reel and the locking member and at least having a portion disposed within the reel coaxially with the reel. The webbing load limiting means limits load exerted on the webbing when the rotation of the locking member is stopped in the webbing unwinding direction and the reel rotates for a predetermined amount in the webbing unwinding direction relative to the locking member. The webbing load limiting means includes a ring-like shear plate disposed between the reel and the locking member. The shear plate has a circumferential shearing portion extending in the circumferential direction. The circumferential shearing portion prevents the reel from rotation relative to the locking member when exerted shearing load is less than a preset shearing load, and is plastically deformed and sheared to allow the reel to rotate relative to the locking member when the exerted shearing load exceeds the preset shearing load, in a period between the time when the rotation of the locking member is stopped in the webbing unwinding direction and the time when the rotation of the reel is stopped in the webbing unwinding direction relative to the locking member.
A seat belt retractor of the second aspect comprises, at least, a reel for winding up a webbing; locking means having a locking member, rotation of which in the webbing unwinding direction is stopped in the event of emergency; and webbing load limiting means rotationally connecting the reel and the locking member and at least having a portion disposed within the reel coaxially with the reel. The webbing load limiting means limits load exerted on the webbing when the rotation of the locking member in the webbing unwinding direction is stopped and the reel rotates for a predetermined amount in the webbing unwinding direction relative to the locking member. The webbing load limiting means includes a shear member disposed between the reel and the locking member. The shear member has a predetermined number of circumferential shearing portions extending in the circumferential direction at predetermined intervals. The circumferential shearing portions prevent the reel from rotation relative to the locking member when exerted shearing load is less than any one of respective preset shearing loads, and are plastically deformed and sheared to allow the reel to rotate relative to the locking member when the exerted shearing load exceeds the respective preset shearing loads, in a period between the time when the locking member is stopped from rotation in the webbing unwinding direction and the time when the reel is stopped from rotation in the webbing unwinding direction relative to the locking member.
In the seat belt retractor of the third aspect, the circumferential shearing portions are sequentially sheared in such a manner that first some of the circumferential shearing portions are sheared to rotate the reel relative to the locking member and, after that, some of the remainder of the circumferential shearing portions are sheared.
In the seat belt retractor of the fourth aspect, the respective preset shearing loads of the circumferential shearing portions are set to be different from each other or to be equal to each other, or set such that some of these are equal to each other.
In the seat belt retractor of the fifth aspect, the shear member is composed of a ring-like shear plate.
In the seat belt retractor of the sixth aspect, the ring-like shear plate comprises at least one arcuate outward or outer protrusion which is formed around the outer periphery thereof to protrude in the radial direction and extend in the circumferential direction, and at least one arcuate inward or inner protrusion which is formed around the inner periphery thereof to protrude in the radial direction and extend in the circumferential direction. The reel is formed with a recess in which one of the arcuate outward protrusion and the arcuate inward protrusion is fitted and which is engageable with the one in the webbing unwinding direction. The locking member is fitted in the other of the arcuate outward protrusion and the arcuate inward protrusion and is engageable to the other in the webbing unwinding direction. The circumferential shearing portion is composed of the arcuate outward protrusion or the arcuate inward protrusion.
The seat belt retractor having the aforementioned structure according to the first aspect has limited load characteristics providing two kinds of limited loads, i.e. the limited load by the circumferential shearing portion of the shear plate and the limit load by the webbing load limiting means. Accordingly, the seat belt retractor of the first aspect can effectively absorb impact load exerted on a vehicle occupant as compared to the conventional seat belt retractor having limited load characteristics providing one kind of flat limited load. Especially, the respective timings of two kinds of limited loads can be freely set such that the action of the circumferential shearing portion is conducted in a period between the time when the locking member is stopped in its rotation in the webbing unwinding direction and the time when the reel is stopped in its rotation in the webbing unwinding direction relative to the locking member.
In addition, the seat belt retractor of the second aspect has multi-stage limited load characteristics providing plural kinds of limited loads, i.e. the limited loads by the predetermined number of circumferential shearing portions of the shear member and the limit load by the webbing load limiting means. Accordingly, the seat belt retractor of the second aspect can effectively absorb impact load exerted on the vehicle occupant as compared to the conventional seat belt retractor having limited load characteristics providing one kind of flat limited load. Especially, the respective timings of various kinds of limited loads can be freely set such that the actions of the predetermined number of circumferential shearing portions are conducted in a period between the time when the locking member is stopped in its rotation in the webbing unwinding direction and the time when the reel is stopped in its rotation in the webbing unwinding direction relative to the locking member.
In the invention, the above effects can be obtained just by interposing a ring-like shear plate which can be easily configured and processed between the reel and the locking member. Therefore, the assembly is easy and the cost is low.